In recent years, much research has been directed to the development of "reductive" projection-exposure apparatus employing a charged particle beam (CPB). By "reductive" is meant that the image formed on the sensitive substrate (e.g., semiconductor wafer) is smaller than the corresponding pattern defined by the mask, and "transfer" as used herein means the imprinting of a mask pattern on the surface of a sensitive substrate by irradiating a CPB through the mask pattern (or a portion thereof) and projecting a focused image of the mask pattern (or a portion thereof) onto the substrate surface for exposure. Image projection is erformed using a CPB "projection system" comprising lenses and other components capable of refracting, bending, and/or deflecting the CPB as required to form the projected image. Such research and development have been particularly directed to improving the resolution of the transferred pattern and improvements in throughput (productivity rate).
It is possible with a conventional CPB projection-exposure apparatus (as well as a conventional optical projection-exposure apparatus) to perform projection transfer in a "batch" manner. In "batch" transfer, the pattern of one complete "die" or sometimes of multiple dies is imprinted onto the substrate surface during a single exposure. A "die" corresponds to the pattern for one of a, typically, plurality of similar circuit patterns exposed onto the surface of a wafer, wherein each exposed die will ultimately become a separate device.
Unfortunately, it is extremely difficult to manufacture a mask suitable for batch transfer using a CPB transfer apparatus. In addition, it is difficult to adequately control aberrations of a CPB projection system having a field of view sufficiently large for transferring a complete die in a single exposure.
Thus, recent attention has been concentrated on developing CPB transfer apparatus that utilize a "divided" transfer system. In such a system, the mask pattern is divided into multiple "subfields" each representing a portion of the overall pattern to be transferred to each die. Typically, one exposure is required to transfer each subfield on the mask to a corresponding "transfer subfield" on the substrate. The subfields are normally exposed in an ordered sequential manner. Thus, transferring a single die requires multiple exposures. Divided transfer permits the imprinting of patterns that are simply too large to transfer, by CPB, as a single-die exposure.
With a divided transfer system, image-forming characteristics of the CPB projection system, such as distortion and astigmatism of the image projected onto the substrate surface, can be corrected in each subfield before the transfer operation. This allows each subfield exposure to be made with excellent resolution and positional accuracy on the overall die. However, in order to correct the image-forming characteristics in each subfield in this way, the CPB projection system must typically be accurately evaluated before exposure.
Under normal circumstances, it is possible to resolve, using a CPB, rectangular patterns of various sizes. CPB projection-transfer apparatus are known in which the transverse profile of the CPB can be varied ("variable-shape CPB") . This is typically accomplished by directing the CPB through a two-stage aperture and then through the CPB projection system. By making an appropriate adjustment of the transverse profile of the beam, projected images of variable size can be projected onto the sensitive substrate. Particularly with such apparatus, it is important that the image-forming characteristics of the CPB projection system be accurately evaluated to ensure that patterns are projected onto the substrate with high precision.
Conventionally, image-forming characteristics of an electron-beam projection-optical system are measured by projecting an image of an evaluation pattern onto a marker using the CPB projection system. Electrons reflected from the marker are detected. The marker and the evaluation-pattern image are comparatively scanned. Certain characteristics of the CPB projection system are determined from dimensional intensity variations of the reflected electrons. The marker typically comprises a pattern having features defined by a deposit of fine particles made from a material having a high atomic number (such as gold). The particles are adhered to a substrate made from a material having a low atomic number.
Unfortunately, the reflected-electron signals from the fine particles are weak. Also, electrons are also usually reflected from the substrate; the resulting background "noise" is quite strong, which results in a low signal-to-noise (S/N) ratio of the detection signal. The low S/N ratio, in turn, makes it difficult to evaluate the image-forming characteristics of the CPB projection system with sufficiently high accuracy.
When it is necessary to evaluate a CPB projection system for use in a divided-transfer CPB microlithography apparatus, the background noise normally is too high to provide an adequate S/N ratio of the detection signal.